This invention pertains to microlithography (projecton-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate using an energy beam). Microlithography is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, micromachines, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam, such as an electron beam or ion beam, as the energy beam. Even more specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods exhibiting reduced aberrations such as image displacement caused by temperature changes in the column containing the CPB optical system of such apparatus.
Advancement of microelectronics technology has been accompanied by a relentless drive toward increased miniaturization and higher density of circuit integration. Higher integration and circuit density require correspondingly smaller linewidths of constituent circuit elements. Currently, the required linewidths of integrated circuits are so small that optical microlithography (microlithography performed using ultraviolet light) often cannot provide the necessary pattern resolution. This situation has led to investigations into alternative microlithography technologies offering prospects of substantially better resolution than obtainable with optical microlithography.
One alternative microlithography technology receiving considerable attention is charged-particle-beam (CPB) microlithography, which utilizes a charged particle beam (e.g., electron beam or ion beam) as a pattern-transfer energy beam. Several CPB microlithography approaches have been the subject of intensive research and development effort, and each approach has respective advantages and disadvantages. One approach offering prospects of reasonably good resolution and throughput is the so-called xe2x80x9cdivided reticlexe2x80x9d projection-transfer approach.
Divided-reticle projection transfer involves dividing the pattern, as defined on a reticle, into multiple individual exposure units usually termed xe2x80x9csubfields.xe2x80x9d Each subfield is exposed individually by projection onto a respective region on the wafer. The subfield images are transferred to the wafer so that, after exposing all the subfields, the subfield images are xe2x80x9cstitchedxe2x80x9d together in a contiguous manner to form the entire chip pattern. As each subfield is exposed, corrections can be made to achieve proper focus and reduction of aberrations (e.g., distortion) for the particular subfield. Divided-reticle projection transfer allows exposures to be made over an optically wide field with much better resolution and accuracy than obtainable by projection-exposing the entire reticle in one shot using a charged particle beam.
Certain aspects of divided-reticle projection transfer are shown in FIGS. 9 and 10. FIG. 9 depicts a wafer showing the intended sites of multiple xe2x80x9cchipsxe2x80x9d or xe2x80x9cdies.xe2x80x9d As exposed, each chip comprises multiple xe2x80x9cstripes,xe2x80x9d and each stripe comprises multiple subfields arranged in rows. This same divided arrangement of stripes and subfields is used to define the pattern on the reticle. FIG. 10 depicts an actual exposure. For exposure, the reticle and wafer are mounted on respective stages (not shown but well understood in the art) configured to move the reticle and wafer horizontally (in the figure) as required for exposure. During exposure of a stripe (a portion of which is shown), the reticle stage and wafer stage both move along the longitudinal line of the respective stripes. Movements of the reticle and wafer are at constant respective velocities (but in opposite directions) in accordance with the demagnification ratio of the projection lens. Meanwhile, the charged particle beam incident on the reticle (the beam upstream of the reticle is termed the xe2x80x9cillumination beamxe2x80x9d and passes through an xe2x80x9cillumination-optical systemxe2x80x9d to the reticle) illuminates the subfields on the reticle row-by-row and subfield-by-subfield within each row. The rows extend perpendicularly to the movement directions of the reticle and wafer. As each subfield is illuminated in this manner, the portion of the illumination beam passing through the respective subfield (now termed the xe2x80x9cpatterned beamxe2x80x9d or xe2x80x9cimaging beamxe2x80x9d) passes through a projection-optical system (including the projection lens) to the wafer.
During exposure of a stripe, to expose the subfields within each row of the stripe in a sequential manner, the illumination beam is deflected at right angles to the movement direction of the reticle stage, and the patterned beam is deflected at right angles to the movement direction of the wafer stage. After completing exposure of each row, the illumination beam is deflected in the opposite direction, as shown in FIG. 10, to expose the subfields in the next row of the stripe. This exposure technique reduces extraneous deflections of the beam and improves throughput.
In a divided reticle, as noted above, each subfield defines a respective portion of the pattern. Usually, each subfield is surrounded by xe2x80x9cstrutsxe2x80x9d (relatively thick structural members that collectively strengthen and provide rigidity to the reticle) and by a xe2x80x9cskirtxe2x80x9d (a thin non-patterned zone adjacent to the struts and surrounding the patterned region of the subfield). The struts extend from non-patterned portions of the reticle located between the subfields. The skirt helps to provide illumination isolation for each subfield to avoid illumination of adjacent subfields whenever a particular selected subfield is being illuminated.
In a conventional CPB projection-microlithography apparatus, a portion located between the reticle 3 and substrate 4 is shown in FIG. 11. Specifically shown are first and second projection lenses 1, 2, respectively, arranged on an optical axis 6. Associated with the first projection lens 1 is a deflector 7 and a ferrite stack 9, wherein the ferrite stack 9 is located in a radial space between the first projection lens 1 and the deflector 7. Associated with the second projection lens 2 is a deflector 8 and ferrite stack 10, wherein the ferrite stack 10 is located in a radial space between the second projection lens 2 and the deflector 8. Also shown is a scattering aperture 5 centered on the axis 6 and located where the beam (xe2x80x9cpatterned beamxe2x80x9d) forms a crossover. An exemplary beam trajectory 11 from the reticle 3 to the substrate 4 is shown.
Each ferrite stack 9, 10 typically comprises alternating rings of non-magnetic ferrite and of high-permeability ferrite stacked atop one another coaxially with the axis 6. Hence, the ferrite stacks 9, 10 are each symmetrical to the same axis as the optical axis 6. The inside diameter, outside diameter, thickness, etc., are determined so as to satisfy the desired parameters provided. In some configurations, the non-magnetic ferrite is not included in the ferrite stack, but rings of the non-magnetic ferrite generally are included with the rings of high-permeability ferrite to improve the positional accuracy of the high-permeability ferrite.
The reticle 3 is irradiated by an electron xe2x80x9cillumination beamxe2x80x9d passing through an xe2x80x9cillumination-optical systemxe2x80x9d (not shown but understood to be located upstream of the reticle 3). Hence, the portion shown in FIG. 11 is the xe2x80x9cprojection-optical system.xe2x80x9d A combination of an illumination-optical system and projection-optical system as used in a CPB projection-microlithography apparatus is referred to herein as a CPB optical system. As projected onto the substrate 4, the image of the illuminated portion of the reticle 3 is xe2x80x9cdemagnifiedxe2x80x9d by which is meant that the image as formed on the substrate 4 is smaller (usually by a factor that is the reciprocal of an integer) than the corresponding reticle portion illuminated by the illumination beam.
By xe2x80x9csensitivexe2x80x9d is meant that the substrate 4 is coated with a suitable xe2x80x9cresistxe2x80x9d material that responds, to exposure by the patterned beam, in an image-forming way. I.e., when exposed to the patterned beam, a latent image of the image carried by the patterned beam is formed in the resist.
The scattering aperture 5, situated between the projection lenses 1,2, blocks downstream propagation of charged particles of the patterned beam that were scattered as they passed through the reticle 3. The deflector 7, when appropriately energized, urges the patterned beam, propagating from a specified location on the reticle 3, to propagate downstream of the reticle 3 along the desired trajectory 11 and through the scattering aperture 5. The deflector 8, when appropriately energized, urges the patterned beam passing through the scattering aperture 5 to a specified location on the substrate 4 where the image is formed. The deflectors 7, 8 also reduce distortion and aberrations of the patterned beam.
The ferrite stacks 9, 10 serve, inter alia, to prevent the creation of eddy currents in the metal constituting the lenses 1,2, respectively. The eddy currents arise from the effects of alternating-current (AC) magnetic fields created by the deflectors 7, 8, respectively. More specifically, the ferrite stacks 9, 10 shield the patterned beam from unintended magnetic fields, and can be used to adjust the profile of the respective lens magnetic fields that are not quite optimal due to manufacturing errors in the respective lenses. The ferrite also ensures that the magnetic fields created by the lenses 1, 2 are more efficiently formed and utilized in the region of the optical axis 6. Also, ferrite is used in the cores of deflectors 7, 8.
In divided-reticle CPB microlithography, high throughput can be achieved by using a relatively high beam current in the illumination-optical and projection-optical systems. However, higher beam currents tend to generate more xe2x80x9cCoulomb effects.xe2x80x9d A Coulomb effect is caused by the mutual repulsion between individual charged particles of the beam, which tends to reduce focus and image sharpness. Coulomb effects can be reduced by accelerating the charged particle beam at a higher voltage or reducing the axial distance between the reticle and the substrate. In either instance, however, to achieve the desired beam trajectory, the electrical current supplied to lenses and deflectors in the CPB optical system must be increased correspondingly. Increasing the electrical power supplied to lenses and deflectors causes these components to exhibit correspondingly more heating.
In divided-reticle CPB projection-microlithography, throughput is increased by reducing the number of times the substrate is xe2x80x9csteppedxe2x80x9d mechanically to a new position for exposure of a row of subfields, which can be achieved by widening the lateral range of beam deflection. I.e., starting and stopping motions of the wafer stage and reticle stage consume time. Widening the lateral range of beam deflection reduces the number of times that this mechanical stepping must be performed for exposure of each die, thereby reducing the time required for exposure of each die. By reducing the exposure time per die, the time required to expose a wafer is reduced and throughput is increased correspondingly. However, the lateral distance in which the beam can be deflected is proportional to the excitation current supplied to the responsible deflector, and increasing the excitation current typically results in correspondingly more heating of the deflector.
The heating of lenses and deflectors, as discussed above, from actions taken to increase throughput is significant and can affect the quality of the microlithographic exposure in an adverse manner. For example, if heating of a lens or deflector of the CPB optical system occurs while making an actual exposure, an undesired change can occur in the shape of the magnetic field produced by the lens or deflector. The magnitude of change depends upon the temperature increase experienced by the lens or deflector and the material from which the lens or deflector is made. Even though conventional electromagnetic lenses are designed to minimize temperature fluctuations, undesired temperature fluctuations still occur, which prevent conventional CPB microlithography apparatus from providing optimal resolution and image quality.
From investigations into the causes of residual temperature fluctuations of lenses and deflectors used in the charged-particle-beam (CPB) optical system, it has been discovered that a previously overlooked factor was responsible. Namely, the ferrite stacks 9, 10, situated between deflectors and lenses, are subject to temperature changes. These temperature changes normally are accompanied by changes in the magnetic flux density along the optical axis of the CPB optical system and can cause increased image displacement and loss of focus at the substrate.
A first aspect of the invention is directed, in the context of a CPB exposure method, to methods for reducing image displacements caused by temperature fluctuations in the lens assembly. In an embodiment of such a method, an allowable range of image displacement is determined. The image displacement is that caused by the ferrite stack experiencing a temperature change, from its normal operating temperature, within a specified temperature range during exposure. Based on a ferrite-composition parameter and/or a ferrite-fabrication parameter, a ferrite is selected for use in the ferrite stack. The selected ferrite is one that exhibits, with a change in ferrite temperature from a normal operating temperature within the specified temperature range, a sufficiently low change in permeability. The change in permeability is sufficiently low so that, if the ferrite experiences a temperature change from the normal operating temperature within the specified temperature range during exposure, the image displacement is within the allowable range. The lens assembly is configured with a ferrite stack including the selected ferrite.
According to another aspect of the invention, CPB exposure apparatus are provided. An embodiment of such an apparatus comprises a lens assembly comprising an electromagnetic lens and/or a deflector arranged relative to an optical axis. The lens assembly includes a ferrite stack situated between the optical axis and the lens and/or deflector. The ferrite stack comprises ferrite exhibiting a change in permeability, with a change in temperature from a normal operating temperature of the ferrite stack, resulting in an image displacement, caused by a change in permeability of the ferrite, of 1 nm or less per 0.01xc2x0 C. temperature change of the ferrite stack from the normal operating temperature during exposure.
According to another aspect of the invention, CPB optical components are provided for a CPB optical system for transferring an image of a pattern to a substrate. The CPB optical components include a unit of ferrite exhibiting first and second initial magnetization curves for different temperatures. The initial magnetization curves intersect each other at a preselected magnetic field intensity.
In one embodiment, the unit of ferrite exhibits a saturation magnetic flux density that decreases with an increase in ferrite temperature relative to a normal operating temperature of the ferrite. Also, a slope of the initial magnetization curve at the normal operating temperature of the ferrite is positive. In other words, a secondary peak of initial permeability has an apex at a temperature higher than the normal operating temperature of the ferrite.
In another embodiment, the unit of ferrite exhibits (a) a saturation magnetic flux density that decreases with an increase in ferrite temperature relative to a normal operating temperature of the ferrite, and (b) a maximum permeability that increases with an increase in ferrite temperature relative to the normal operating temperature of the ferrite.
In yet another embodiment, the unit of ferrite exhibits a saturation magnetic flux density that increases with an increase in ferrite temperature relative to a normal operating temperature of the ferrite. Also, a slope of the initial magnetization curve at the normal operating temperature of the ferrite is negative. In other words, a secondary peak of initial permeability has an apex at a temperature lower than the normal operating temperature of the ferrite.
In yet another embodiment, the unit of ferrite exhibits (a) a saturation magnetic flux density that increases with an increase in ferrite temperature relative to a normal operating temperature of the ferrite, and (b) a maximum permeability that decreases with an increase in ferrite temperature relative to the normal operating temperature of the ferrite.
Any of the CPB optical components summarized above can include an electromagnetic lens arranged relative to an optical axis, and a ferrite stack situated adjacent the electromagnetic lens. When energized, the electromagnetic lens produces a DC magnetic field at the optical axis. The ferrite stack comprises the unit of ferrite. In this configuration, the preselected magnetic field intensity at which the first and second initial magnetization curves intersect desirably is nearly the same as the intensity of the DC magnetic field. This configuration further can include a deflector arranged relative to the electromagnetic lens and the optical axis. The deflector when electrically energized produces an AC magnetic field at the optical axis, and the ferrite stack is situated adjacent the deflector and the electromagnetic lens. In the configuration including both a lens and deflector, the preselected magnetic field intensity at which the first and second initial magnetization curves intersect is nearly the same as a sum of the intensity of the DC magnetic field and the intensity of the AC magnetic field. Furthermore, the deflector and the electromagnetic lens can be operated to cause a B-H curve of the ferrite to exhibit a local loop, wherein the center of the local loop corresponds to a magnetic field intensity that exhibits substantially no change with changes in the ferrite temperature relative to the normal operating temperature of the ferrite.
Yet another embodiment of the CPB optical component includes a deflector arranged relative to the optical axis. When electrically energized, the deflector produces an AC magnetic field at the optical axis. The component also includes a ferrite stack, situated adjacent the deflector, that includes the unit of ferrite. In this configuration, the preselected magnetic field intensity at which the first and second initial magnetization curves intersect desirably is nearly the same as the intensity of the AC magnetic field.
According to another aspect of the invention, CPB optical systems and CPB exposure systems are provided that comprise any of the CPB optical components summarized above.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.